1. Field of the Invention
The present invention relates to a sand core and a method of making a sand core.
2. Description of the Prior Art
Molds for casting molten metals comprise several mold members working together to define the internal and external shape of the casting. Such mold members include core members for forming and shaping the interior cavities of the casting. The core members are typically made by mixing sand with a binder, introducing the binder-sand mix into a mold containing a pattern for shaping the sand-binder mix to the desired shape for making the metal casting, and curing/hardening the binder in the pattern mold to harden the binder and to fix the shape of the mold-forming material.
Gelatin has been used as a binder for the sand. Gelatin is desirable because it is water soluble, environmentally benign, and less costly than synthetic resins used in many sand-binder systems. In addition, less heat is required to break the bonds of the gelatin""s protein structure to thermally degrade the binder than is required for the synthetic resin binders. As a result, in the case of mold members which are cores, the gelatin binders break down readily from the heat of the molten metal, and thereby permit ready removal of the core sand from the casting with a minimum of additional processing such as shaking or hammering. Moreover, because the gelatin is water soluble, any sand that is not removed from the casting mechanically can be readily washed therefrom with water. Solubility of gelatin also permits ready washing of the binder from the sand for recycling and reuse of the sand to make other mold members and thereby eliminate the cost of using new sand for each mold.
Gelatin is a protein material obtained by the partial hydrolysis of collagen, the chief protein component of skin, bone, hides and white connective tissue of animals and is essentially a heterogeneous mixture of polypeptides comprising amino acids including primarily glycine, proline, hydroxyproline, alanine, and glutamic acid. Gelatin is sold commercially as a by-product of the meat producing industry. xe2x80x9cDryxe2x80x9d commercial gelatin actually has about 9% to about 12% by weight water entrained therein, and is an essentially tasteless, odorless, brittle solid having a specific gravity between about 1.3 and 1.4. Gelatins have a wide range of molecular weights varying from about 15,000 to above 250,000, but can be separated one from another by suitable fractionation techniques known to those skilled in the art. Gelatins are classified by categories known as xe2x80x9cBloomxe2x80x9d ratings or numbers. The Bloom rating or number is determined by the Bloom test which is a system for rating the strength of gels formed from different gelatins. Gelatins having high Bloom ratings/numbers comprise primarily polypeptides with higher average molecular weights than gelatins having lower Bloom ratings/numbers. The Bloom rating/number is determined by evaluating the strength of a gel formed from the gelatin. Typically, the viscosity of the gelatin is measured at the same time as the Bloom rating/number by using the same gelatin sample as is used for the Bloom test. The viscosity of the gelatin is generally correlated to the Bloom rating/number. In other words, as the Bloom rating/number increase so does the viscosity.
U.S. Pat. No. 5,320,157 to Siak et al. teaches an improved gelatin binder for sand core members wherein a ferric compound is incorporated into the binder. The ferric compound enhances the thermal breakdown of the binder during the casting process thereby simplifying removal of the spent sand from the cast article. A typical method for forming a core mold is disclosed.
U.S. Pat. No. 5,582,231 to Siak et al. requires chilling the gelatin coated sand with or without rehydration to ambient temperatures or below before blowing the gelatin coated sand into the mold. This chilling step is performed so that the gelatin coating will gel when it is hydrated and the sand will be less sticky. The chilling step can require expensive cooling systems in metal foundries where the environment is typically warm due to the presence of molten metals. When the hydrated, coated sand temperature is above ambient temperatures, the gelatin gel coating melts and the sand is sticky, which hinders the flow of the sand. However, even if the hydrated, coated sand is chilled, it still does not flow as well as dry sand or even sand coated with phenolic urethane (cold box) resin.
In another patent to Siak et al., U.S. Pat. No. 5,749,409, a method for providing a topcoat of refracting particles to a foundry core formed from gelatin coated sand is disclosed. An organic waterproof layer is applied to the surface of the core and the refractory particles are then applied as an aqueous suspension. The waterproof layer protects the core from deterioration resulting from water in the aqueous suspension. The core is formed according to the description in U.S. Pat. No. 5,320,157.
U.S. Pat. No. 2,145,317 to Salzberg teaches the use of a mixture of a soluble proteinaceous material such as gelatin and a crystallizable carbohydrate as a binding material for making baked foundry cores. The method of forming core molds is discussed in general terms.
A method for removal of a sand core from a molded product with water is taught in U.S. Pat. No. 5,262,100 to Moore et al. This patent discloses binder materials including carbohydrates and proteins such as gelatin. A general process for forming a core mold is described.
U.S. Pat. No. 5,580,400 to Anderson et al. discloses packaging materials formed from fiber reinforced aggregates held together by organic binders including gelatin. Various methods of forming molded articles are disclosed.
In a preferred embodiment method of making a molded article for use in a casting process, sand particles are mixed with protein and water to effect a coating of protein on the sand particles. The protein coated sand particles are then dried and blown into a mold without active cooling. Steam is then passed through the protein coated sand particles to hydrate and melt the protein, thereby forming bonds between contiguous sand particles to form a molded article. Hot, dry air is then passed through the molded article to harden the protein bonds between contiguous sand particles.
In a preferred embodiment method of making a sand core, sand particles are mixed with gelatin and water while supplying heat, wherein the heat melts the gelatin to effect a coating of gelatin on the sand particles and dries the gelatin coated sand particles. The dry, gelatin coated sand particles are blown into a mold without active cooling, and then steam is passed through the gelatin coated sand particles to hydrate and melt the gelatin, thereby forming bonds between contiguous sand particles to form a molded article. Hot, dry air is then passed through the molded article to harden the gelatin bonds between contiguous sand particles.
In another preferred embodiment method of making a sand core, sand particles are mixed with gelatin and water to create a mixture. Heat is supplied to the mixture to effect a coating of gelatin on the sand particles and to dry the water thereby drying the mixture. The dried mixture is then ground thereby making the mixture free flowing, and the dry, gelatin coated sand particles are blown into a mold. Steam is passed through the gelatin coated sand particles to hydrate and melt the gelatin, thereby forming bonds between contiguous sand particles to form a molded article. Hot, dry air is passed through the molded article to harden the gelatin bonds between contiguous sand particles.
In another preferred embodiment method of making a sand core, sand particles are heated to above 40xc2x0 C. and then mixed with gelatin and water, wherein the heated sand particles melt the gelatin thereby coating the sand particles with gelatin. The gelatin coated sand particles are then dried and blown into a mold. Steam is passed through the gelatin coated sand particles to hydrate and melt the gelatin, thereby forming bonds between contiguous sand particles to form a molded article. Hot, dry air is passed through the molded article to harden the gelatin bonds between contiguous sand particles.
In another preferred embodiment method of making a sand core, sand particles are mixed with protein and water to effect a coating of protein on the sand particles. The protein coated sand particles are then dried and blown into a mold. The protein coating the sand particles is then rehydrated within the mold thereby forming bonds between contiguous sand particles to form a molded article. Hot, dry air is then passed through the molded article to harden the protein bonds between contiguous sand particles.